In the related art, this kind of power converting apparatus is provided with a DC-DC converter circuit which steps up or down voltage from a DC power supply, a DC-AC converter circuit (inverter) which converts output voltage (hereinafter, referred to as intermediate voltage) of the DC-DC converter circuit into AC, and a control unit which controls the DC-DC converter circuit and the DC-AC converter circuit.
In such a power converting apparatus, there is a problem that, if input voltage (intermediate voltage) to the DC-AC converter circuit is insufficient with respect to a peak value of voltage that the DC-AC converter circuit should output, an output current of the DC-AC converter circuit is distorted near the peak value. On the other hand, if the intermediate voltage is too high, switching loss of each of a device in the DC-DC converter circuit and a device in the DC-AC converter circuit increases, whereby conversion efficiency is impaired.
For this reason, it is preferred that input voltage (intermediate voltage) to the DC-AC converter circuit is higher than the peak value of voltage which the DC-AC converter circuit outputs by an amount of a margin (an amount influenced) because of, for example, an influence of the amount of current that the DC-AC converter circuit outputs to the load (the load and a distribution system in a case in which the DC-AC converter circuit is used as a grid interconnection apparatus) and an influence of voltage drop in the device of the DC-AC converter circuit.
The power converting apparatus needs to determine the peak value of voltage that the DC-AC converter circuit outputs in consideration of voltage which the load requires and the margin because the peak value is affected by voltage that the load requires.
As a related art example of such a power converting apparatus, a system in which voltage (grid voltage) of the distribution system is detected and sets a value obtained by adding a predetermined margin to the peak value of the grid voltage as an intermediate voltage target value is proposed.
However, an appropriate margin between the intermediate voltage and the peak value of the grid voltage depends on, for example, voltage drop of a main circuit component which exists in the section and on the length of dead time during which short-circuit between upper and lower arms, and thus changes due to, for example, the amount of output current and variation in the main circuit component.
Therefore, in the power converting apparatuses disclosed in Patent Literatures 1 and 2 which process the margin as a constant value, it has not been possible to properly control the intermediate voltage.